The invention relates to digital transmission systems and more particularly to a system for enabling at the receiving end to extend a speech signal received in a narrow band, for example the telephony band (300-3400 Hz) into an extended speech signal in a wider band (for example 100-7000 Hz).
Most current telecommunication systems transmit a speech bandwidth limited to 300-3400 Hz (narrow band speech). This is sufficient for a telephone conversation but natural speech bandwidth is much wider (100-7000 Hz). Actually, the low band (100-300 Hz) and the high band (3400-7000 Hz) are important for listening comfort, speech naturalness and for better recognizing the speaker voice. The regeneration of these frequency bands at a phone receiver would thus enable to strongly improve speech quality in telecommunication systems. Moreover, during a phone conversation, speech is often corrupted by background noise especially when mobile phones are used. Also, the telephone network may transmit music played by switchboards. Therefore, the system that generates the low band and high band should both fit as much as possible to speech and should allow to reduce noise and improve music subjective quality.
The U.S. Pat. No. 5,581,652 describes a Code book Mapping method for extending the spectral envelope of a speech signal towards low frequencies. According to this method, low band synthesis filter coefficients are generated from narrow band analysis filter coefficients thanks to a training procedure using vector quantization as described in the article by Y. Linde, A. Buzo, R. M. Gray: xe2x80x9cAn algorithm for Vector Quantizer Designxe2x80x9d, IEEE Transactions on Communications, Vol. COM-28, No 1, January 1980. The training procedure allows to compute two different code books: an extended one for the extended frequency band and a narrow one for the narrow band. Said narrow code book is computed from the extended code book using vector quantization so that each vector of the extended code book is linked with a vector of the narrow band code book. Then the coefficients of the low band synthesis filter are computed from these code books.
However, this method presents some drawbacks, which are responsible for the production of a rattling background sound. First the number of synthesis filter shapes is limited to the size of the code books. Second the extracted vectors in the extended band are not very correlated with the vectors obtained from the linear prediction of the narrow band speech signal. Another method called extension matrix was thus developed in order to improve signal quality at the receiving end.
It is an object of the invention to provide a method for extending at the receiving end a narrow band speech signal into a wider band speech signal in order to increase signal naturalness and listening comfort which yields to a better signal quality. The invention is particularly advantageous in telephony systems.
In accordance with the invention, the received speech signal is detected with respect to a specific speech characteristic before an extension matrix is applied to the signal, said extension matrix having coefficients depending on said detected characteristic.
In a preferred embodiment of the invention, said specific characteristic called voicing relates to the detected presence of voiced/unvoiced sounds in the received speech signal which can be detected by known methods such as the one described in the manual xe2x80x9cSpeech Coding and Synthesisxe2x80x9d, by W. B. Kleijn and K. K. Paliwal, published by Elsevier in 1995. Then the matrixes are computed from a data base, said data base being split with respect to the detected voicing, by applying an algorithm based on Least Squared Error criterion on Linear Prediction Coding (LPC) parameters as described by C. L. Lawson and R. J. Hanson, in xe2x80x9cSolving Least Squares Problemsxe2x80x9d, Prentice-Hall, 1974, or based on the Constrained Least Square method described in xe2x80x9cPractical Optimizationxe2x80x9d by P. E. Gill, W. Murray and M. H. Wright published by Academic Press, London 1981.